Direct current motors are normally started from a standstill through the employment of starting resistor banks which are successively switched out of the circuit or alternatively switched into the circuit under control of mechanical switch units. Even disregarding the fact that such switching parts wear out, start-up operation of a d.c. motor is not smooth and continuous, due to the afore-mentioned switching operations.
It is now known from the article "Elektrische Bahnen" 38th Edition (1967), Issue No. 10, that in a compound-wound motor, the armature current is regulated to a desired value and, after obtaining a particular speed, a further increase in velocity is obtained by field reduction. From the compound-wound circuit it becomes clear that the d.c. motor is operated also in the lower speed rate with a series-wound characteristic as the excitation follows the armature current.
On the other hand, it is also known from D.E.-A.S. No. 19 26 980, to operate a shunt-wound motor in the lower speed range so that the armature current is controlled to a given rated value or reference input signal, the excitation thereby being varied together with the armature current actual value so that motor operation follows the operating characteristic of series-wound d.c. motors. If the armature current can no longer follow the rated signal when the motor reaches a particular speed, the excitation will be decreased. Thereby the actual value of the armature current can again be increased to the rated value i.e. the input reference signal.
It is further known that series-wound motors are not suitable for regenerative braking due to the dependancy of armature current and excitation. The efforts to realise such operation with a compound-wound d.c. motor are also relatively difficult due to the dependancy of armature current and excitation here too. Series-wound generators (in the regenerative braking operation the direct current motor operates as a generator) as a practical matter, are not used due to the above-mentioned reason. However although compond-wound generators are used, it is still important to take into consideration the fact that the quadrant change of a motor when operating as a generator (during braking) is also relatively more complicated in compond-wound circuits than is the case with pure shunt-wound circuits.
The present invention solves the following problems by operating a d.c. motor such that the advantageous characteristics of both the series-wound and the shunt-wound motor are both utilized within a unique circuit. Before further explaining the invention some definitions of signals in a general automatic control loop are given. These definitions are for example taken from IEEE-Transactions, Vol Pas-88, August 1969:
The input signal to the control system is "the reference input signal" or may be referred to as "the rated value signal". PA1 This signal is compared with either the "control-led (output) signal" directly if the feedback circuit is just a unity feedback, or with a signal derived from the controlled signal by means of the feedback circuit. PA1 The "controlled signal" may also be said the "actual value signal" with respect to the "rated value signal". PA1 The result of the comparison between actual and rated value is the "control deviation". PA1 This "deviation" acts to adjust the "actuating signal" which latter actuates the controlled or actual signal so that the latter reaches the rated value.
The method of the present invention is thus characterized by the fact that a d.c. motor is operated in such a way that, depending upon the operational conditions it is controlled to follow a shunt-wound characteristic or a series-wound characteristic of torque and revolution.